We are using mutant enzymes obtained by recombinant DNA technology to examine the relationship between the structural and kinetic properties of DNA polymerases and their fidelity. Emphasis is on polymerases for which X-ray crystal structure information is or may soon become available. We have established the processivity and error specificity of exonuclease-deficient Klenow polymerase at two reaction pHs. We extended the anlaysis to three mutants in which a tyrosine residue at position 766, in the "O helix" of the polymerase active site, was changed to an alanine, serine or phenylalanine, and to additional active site residues involved in catalysis and metal binding. Several of these mutants have altered processivity and altered fidelity, some having mutator and others antimutator phenotypes. The data suggest that dNTP and metal binding residues are important for determining base selectivity, but (to date) not template-primer misalignment-initiated errors. We are presently examining mutant derivatives of the HIV-1 reverse transcriptase, in which we have mutated the "thumb" residues known to be important for binding double-stranded template-primer. We have also begun to establish the fidelity of the wild-type T4 and T7 DNA polymerase, as well as their exonuclease-deficient partners differing by one and two conserved amino acids, respectively. Analyses of recombinant DNA polymerases for which considerable genetic and/or kinetic data are available should improve our understanding of accurate DNA synthesis and how fidelity is affected by DNA adducts.